[python] don't attempt systemdep install if no deps

[vlinkz]

Why

If no system dependencies are found, don't to toml-editor nop.

What changed

Check if no system dependencies need to be installed for python packages.

Test plan

[vlinkz]

Hm this is not actually fixing it in the right place. I'll fix in a min, but we should check if no new packages are added, not no packages at all.